This application claims priority of Korea patent Application No. 99-62083, filed on Dec. 24, 1999.
(a) Field of the Invention
The present invention relates to a control method for a diesel engine, and more particularly, to a control method for a diesel engine in which a vehicle speed limit is increased after determining that the vehicle is travelling on a downgrade, thereby enabling the momentum of the downgrade to be used to propel the vehicle.
(b) Description of the Related Art
The main structural elements of a diesel engine for converting heat energy into mechanical energy are not significantly different from those of the gasoline engine. However, the processes of fuel supply and fuel combustion are performed differently in the diesel and gasoline engines. In particular, only air is supplied during the intake stroke, and after compression to a high compression ratio of 15-22:1, resulting in an increase in temperature to roughly 500-600xc2x0 C., fuel injected into the combustion chamber is ignited as a result of the high temperature generated therein (i.e., the fuel undergoes self-combustion).
Accordingly, a fuel injection system is needed in the diesel engine. Also, diesel fuel, which easily self-combusts, must be used. The fuel injection system of the diesel engine includes a fuel tank, a fuel pipe, a fuel supply pump, a fuel filter, an injection pump, a high pressure pipe, and an injection nozzle. Fuel is supplied to the injection nozzle by first passing through these elements in the order listed.
The injection pump is driven by an engine crankshaft to pressurize fuel to a high pressure and inject the fuel into the combustion chamber through the injection nozzle. The fuel is injected into the combustion chamber at a predetermined pressure and at the appropriate time. Together with the shape of the combustion chamber, injection timing, injection period, fuel injection amount, injection state (i.e., spray travel distance or penetration force), distribution state, degree of atomization, etc., these are all important factors in determining the combustion state. A governor and timer are provided in the injection pump to enable the variation of fuel injection amounts and fuel injection timing.
The fuel injection amount and fuel injection timing can be mechanically controlled or electronically controlled using a microcomputer. These parameters are now, for the most part, controlled electronically. Also, much research is being conducted to enable more precise control of fuel injection amount and timing.
Diesel engines are more often found in large vehicles such as buses and trucks. As a safety precaution, a speed limit device is typically provided to prevent the vehicle from travelling over a predetermined speed of, for example, a legal speed limit. That is, when the predetermined speed is exceeded, operation of the accelerator pedal has no effect in that the fuel supply to the engine is cut-off.
In both the vehicle equipped with the mechanical and electronic control of the fuel injection amount and fuel injection timing, it is possible to disengage such control. In the case of the diesel engine that is electronically controlled, it is possible to remove or disconnect the vehicle speed sensor, thereby disenabling fuel injection amount and timing control. However, when the vehicle speed sensor is removed or disconnected, various other electronic controls are also disabled, such as the automatic cruise control.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a control method for a diesel engine in which the vehicle speed limit is automatically increased to a specific level to enable an increase in speed by the operation of the accelerator pedal.
To achieve the above object, the present invention provides a method for controlling a diesel engine comprising the steps of determining if a vehicle is travelling on a downgrade; outputting a predetermined fuel supply cut-off signal when the vehicle is travelling on a downgrade; determining an error between a vehicle speed limit and a present vehicle speed; setting an aiming speed as a first predetermined value; determining an aiming error; determining if the aiming error is greater than 0; setting an enable vehicle speed as a second predetermined value when the aiming error is greater than 0; setting the enable vehicle speed to 0 when the aiming error is less than 0; subtracting the error between the vehicle speed limit and the present vehicle speed from the enable vehicle speed to thereby derive an enable error; determining if the enable error is greater than 0; setting an offset vehicle speed Off_set as a third predetermined value when the enable error is greater than 0; setting the offset vehicle speed to 0 when the enable error is not greater than 0; determining if an accelerator pedal position has reached or is greater than a fourth predetermined value; adding the offset vehicle speed to the vehicle speed limit to thereby obtain a new vehicle speed limit when the accelerator position has reached the fourth predetermined value; determining if the present vehicle speed is less than the new vehicle speed limit; outputting a fuel supply control signal when the present vehicle speed is less than the new vehicle speed limit; and outputting a fuel supply cut-off control signal when the present vehicle speed is greater than the new vehicle speed limit.
According to a feature of the present invention, it is determined if the vehicle is travelling on a downgrade when the vehicle speed exceeds the vehicle speed limit for over a predetermined duration of time.
According to another feature of the present invention, the first and second predetermined values are stored in a pre-installed program and are respectively 6 kph and 4 kph.
According to still another feature of the present invention, when the enable error is greater than 0, it is determined that the vehicle has reached the end of the downgrade.
According to still yet another feature of the present invention, the third predetermined value is stored in a pre-installed program and is 20 kph.
According to still yet another feature of the present invention, the fourth predetermined value is 50%.